This invention relates to the process of deposition of boron carbide semiconductor material, and also to semiconductor devices formed by deposition of a boron carbide film. The invention is more particularly directed to a technique for creating a layer of boron carbide with a boron-to-carbon ratio selected to achieve a suitable semiconductor energy band gap. The invention is also particularly directed to heterojunction semiconductor devices produced by this technique.
Techniques are known for forming boron-rich carbides. These techniques can employ alkanes and heavy boron cage molecules to deposit boron carbide thin films. Plasma-enhanced chemical vapor deposition (PECVD) can be employed to fabricate boron carbide films without resort to high temperatures or high pressures. These technique typically employ a halide of boron, e.g. BCl.sub.3, BBr.sub.3 or BI.sub.3. Most recently boranes, such as nido-decaborane and nido-pentaborane have gained interest, because these compounds are safe and stable, yet produce a vapor pressure of several Torr at room temperature. However, until very recently, only low-resistivity boron carbide materials could be produced, i.e., materials with resistivities on the order of about ten ohm-cm at room temperature. Boron carbide material of this type has an extremely low band gap, and is not suited as a semiconductor material.
At the same time, boron carbide has become an attractive material because of its inherent hardness and durability. Boron carbide, like other boron-containing materials, has been considered for high temperature electronic devices because it retains its useful characteristics at elevated temperatures. For example, boron carbide is know to have a melting temperature of 2350.degree. C., a strength of 50 ksi, a hardness of 2800 kg/mm.sup.2, and a thermal conductivity of 0.22 cal/cm/sec/.degree.C./cm. Diamond and silicon carbide have been investigated because of their good thermal and mechanical characteristics, and because of their wide band gaps. However, these materials have not yet proven cost effective.